Flap doors for the above purposes and devices for the locking and unlocking of such flap doors, are well known in the art in several variants. However, in connection with aircraft, especially commercial aircraft, such devices must meet regulations. Thus, the flap doors must, on the one hand, be capable of being safely locked, while on the other hand, it is necessary to be able to open such a flap door without problems even against an additional closing force exerted, for example, by a person standing on the flap door on the upper deck. Such requirements or regulations apply, for example, for long distance commercial aircrafts in which additional pilots and crew members travel in crew quarters below the passenger deck, whereby the crew quarters are accessible through these flap doors so that one shift of crew members can be exchanged against another shift of such crew members through this flap door. Opening of the flap door from below is possible only if there is no load on the flap door on the passenger cabin side. This is not always assured, since these flap doors are normally installed in the aisle. Thus, it is possible that a passenger or crew member is momentarily blocking the flap door by standing. There is no way of alerting the person of the intended opening.